| In the process of offshore oil and gas development,the high temperature resistance and corrosion resistance of dual-phase stainless steel production equipment under harsh corrosion environment are increasingly required.This paper adopts the different content of rare earth La and Cu(S32205 dual phase stainless steel)to process the test steel,after heat treatment on the immersion corrosion experiment and potentiodynamic anodic polarization experiment,and USES the scanning electron microscope(SEM)and scanning electron microscopy,energy spectrum(EDS),transmission electron microscope(TEM)and transmission electron microscopy,energy spectrum(EDS)combined with test methods for inclusion in the duplex stainless steel and inclusion and the border of the steel substrate was carried out in situ analysis and chemical composition analysis.At the same time,Vickers hardness analyzer and micro-tensile testing machine were used to analyze the effects of different La content on the hardness value,tensile strength,yield strength and elongation of dual-phase stainless steel.The conclusions are as follows:(1)Large size inclusions can be modified by rare earth La,and the effect becomes better with the increase of rare earth content from 0 to 0.01%,0.02% and 0.04%.The negative effect of small size inclusions on the pitting resistance of S32205 duplexed stainless steel is less than that of large size inclusions.The influence of inclusions on the pitting corrosion resistance mainly includes the size,quantity and composition of inclusions.The inclusion of S32205 dual-phase stainless steel is composed of the main type of(Al,Ti,Mg)oxides and the mixed type of(Al,Ti,Mg,Ca,Mo)oxides.The addition of rare earth La to 0.01% can remove the oxide containing Mo.Adding rare earth La to 0.02% can further remove the oxides containing Ca.The addition of rare earth La to 0.04% can remove the oxides containing Al on the previous basis.The addition of rare earth La can significantly increase the pitting potential of S32205dual-phase stainless steel,that is,the corrosion resistance of chloride ions can be greatly improved.The rare earth La can improve the hardness,tensile strength,yield strength and elongation of S32205 dual-phase stainless steel,mainly because the rare earth La plays a strengthening role of fine grain strengthening and the second phase strengthening in the dual-phase stainless steel.(2)The addition of copper has little effect on the composition and morphology of inclusions in S32205 duplex stainless steel,and is independent of the addition of copper content;Copper increases the number of large inclusions in S32205 duplexed stainless steel,which has a great negative effect on its pitting corrosion resistance.The(Ca,Ti)oxides in S32205 dual-phase stainless steel are easy to corrode,(Al,Mg)oxides are corrosion resistant,and(Al,Mg)oxides are generally large size inclusion;The addition of copper promotes the formation of micro-cracks and pitting pits between the steel matrix and the inclusion in S32205 dual-phase stainless steel,and with the increase of the addition of copper,the inclusion will dissolve,and finally lead to the nearly separation of the steel matrix and inclusion.When the inclusion is not separated from the steel matrix,the effect of inclusion on pitting corrosion is small.When the inclusions are almost separated from the steel matrix,the influence of inclusions on pitting corrosion is great.The addition of copper can promote the precipitation of chromium phase in S32205 dual-phase stainless steel and increase the content of chromium oxide,which leads to the increase of the initiation point of pitting corrosion and affects the pitting resistance.The addition of copper has a great negative effect on the pitting potential of S32205 dual-phase stainless steel,that is,the ability of resisting chloride ion corrosion is greatly reduced. |
